In the 2016 referendum, the Borough of Lewisham, of which the constituency is a part, voted to remain in the European Union. Through demographic modelling, the Remain vote in the Lewisham East constituency was estimated as 66% by Number Cruncher Politics and as 65% by Chris Hanretty at Royal Holloway.[7]

The Labour selection was reported in the media as revealing splits in the Labour Party.[27] This was the first by-election contested by the party since Jennie Formby took over as general secretary,[2] which led to speculation that centrist candidates might be excluded.[2][28] The Lewisham East Labour Left caucus group called for a socialist MP who would "campaign for...a left-wing government with Jeremy Corbyn as Prime Minister".[29] On 10 May 2018 Ian McKenzie, chair of the Constituency Labour Party (CLP), emailed members concerned that the National Executive Committee (NEC) was rushing the selection to prevent them having their say.[2][30]

On 14 May the NEC announced a shortlist of Webbe, Dacres, Daby and Sheikh,[37] all women from ethnic minorities.[38] McKenzie was said to be delighted by the inclusion of Dacres and Daby.[27] Sheikh received the endorsement of Momentum, whilst Webbe was endorsed by the trade unions Unite[39] and the Communication Workers Union.[40][41] Daby received the support of Unison.[42] Dacres withdrew on 18 May and endorsed Daby, citing "recently diagnosed health reasons".[43] The remaining candidates expressed differing views over Brexit, with Daby pledging to fight for the UK to remain in the customs union and the single market,[17][44] whilst Sheikh and Webbe expressed their opposition to a second referendum or continued membership of the single market.[44]

Sheikh was temporarily withdrawn from the ballot prior to the selection meeting, following accusations that she had supported a rival political organisation, but was reinstated minutes later.[9] Daby went on to be selected by local members on 19 May, receiving 63% of the vote in the first round, with Sheikh receiving 29% and Webbe 8%.[8][45]

On 22 May, McKenzie, credited with having secured the Labour candidature for Daby which he claimed had "humiliated" Momentum and Unite, was suspended as chair of the CLP and is facing possible disciplinary action over tweets from 2015 and 2016 about Emily Thornberry and ISIS.[46] These were uncovered by left-wing writer Owen Jones, who McKenzie had suggested should keep away from the constituency.[47]

The Liberal Democrats announced their opposition to plans by Lewisham Council to seize the land of Millwall FC's youth academy for redevelopment. Party leader Vince Cable visited the academy in late May; the party's candidate, Lucy Salek, announced that the land should be used for social housing if the redevelopment plans went ahead.[48][49]

On 11 June 2018, three days before the by-election, a vote estimate by the Liberal Democrats on the basis of canvassing data suggested that the party would finish second with 25% of the vote, with Labour on 49% and the Conservatives on 16%.[52]

On 12 June 2018, police shut down a by-election hustings in Catford, leading to one arrest, after it was disrupted by anti-racism protesters led by Stand Up to Racism and Lewisham Anti-Racist Action Group.[53][54][55] The protesters intended to target Anne Marie Waters, who did not attend the event on police advice. Daby and Archer were also absent from the hustings, citing other engagements; Daby had previously refused to share a platform with Waters.[54][55] The protest was criticised by Salek and Reid.[53] Prior to the event, Foxcroft and Damien Egan, the Mayor of Lewisham, had called for Waters to be No Platformed at the hustings.[55]

The result was the largest swing from Labour to the Liberal Democrats since 2004.[57] The Liberal Democrat leader Vince Cable said: "This result sends a message to the Labour leadership that it cannot take pro-European voters for granted."[58]The Guardian and Evening Standard reported the result as a "surge" for the Liberal Democrats, which was widely reported as being related to the party's opposition to Brexit.[59][60] After being elected, Daby reaffirmed her opposition to a "hard" Brexit.[58][61][62]